5. RECENT DEVELOPMENTS IN CLASSIFICATION AND MORPHOLOGY

Recent developments in this field have been
concerned with (1) new major catalogues of standard Hubble types; (2)
slight modifications to existing classification systems; (3) the
identification of
new types of galaxies and improved understanding of older types; (4)
identification
of the major orbit resonances in grand design spirals and ringed
galaxies; (5)
widespread application of electronic detectors with high quantum efficiency
to large numbers of galaxies of many types, and over a wide range of
passbands; and (6) computer classification.

Major catalogues with morphological data continue
to be produced. These are summarized in reviews by Corwin
17
and Buta 13.
The RC3 combined several of these catalogues and other smaller lists
into the
largest database of Hubble morphological type information ever
compiled. The
17,700 types given in this catalogue are based solely on photographic
sources and are on the classification system of de Vaucouleurs
19.
Other major catalogues include the Virgo Cluster Catalogue
5
and the RSA; these give types on Hubble's revised system
44,
with additions and revisions, some described below.

No major new classification systems have been proposed since 1976,
though modifications
to existing systems have been suggested. For example, Kormendy
38
suggested that
tenses be distinguished from rings using their own notation within the
framework
of the de Vaucouleurs revised Hubble system. He suggested denoting inner
lenses
by (1) and outer lenses by (L) in the same classification positions
where inner
and outer rings would be specified. Kormendy also suggested a different
approach
to morphology, the idea of characterizing galaxies in terms of a small
number
of "distinct components" (bars, rings, etc.) rather than a large number
of morphological
"cells". The objective of the approach is to make deductions concerning
secular
evolution from the ways these components might be expected to interact (see
Table 1 of Kormendy
39).
Kormendy suggested that such an approach leads to the
possible conclusion that bars are not permanent features of galaxies but
may
evolve under certain circumstances to a lens. Whether this evolution
actually takes place or not is still uncertain.

Another revision to the classification
systems is the recognition of "dusty E's." The misclassification of
these objects
as S0's is a noteworthy problem of catalogues emphasized by Ebneter,
Djorgovski, and Davis
28
(and references therein; =EDD). The presence of such "features"
in a type of galaxy which was by definition featureless led EDD to
suggest a
more physical classification of E's is now warranted. A lovely montage
and catalogue
of dust-lane ellipticals is provided by Bertola
3.

Sandage and Brucato
48
pointed
out that the original classes called Irr I and Irr II in the Hubble
Atlas are
not satisfactory because they combine widely differing objects into the
same bin, namely "Irr". To distinguish galaxies which are not E, S0, or
S but which
have an amorphous appearance to the unresolved light, sometimes with
imbedded
resolved stars, they proposed the term "amorphous" galaxies. Sandage and
Brucato
emphasize that these objects are similar to, but not precisely like, the
Irr
II's in the Hubble Atlas, and that some similar objects classified as I0 by
de Vaucouleurs may be peculiar spirals or S0's. One of the hallmarks of the
amorphous class is a well-developed early-type absorption spectrum
spread throughout the disk.

In the case of spirals, many
aspects of the "grand design" and "flocculent" spiral morphologies have now
been quantified
31.
These are aspects of spiral structure morphology that are
not directly built into Hubble classifications. Flocculent galaxies lack
bimodal
symmetry and have a spiral-like structure composed only of small pieces
of arms.
Grand design galaxies generally have a two-armed structure and the arms are
longer and more continuous than in flocculent galaxies. To account for
these
differences and for combinations of the two pattern types in many galaxies,
Elmegreen and Elmegreen
30
proposed a system of 10-12 "arm classes" or AC's to
highlight a systematic orderliness of spiral arms. The AC's are not exactly
the same as van den Bergh luminosity classes because they emphasize
symmetry and arm length, rather than arm contrast.

The identification of the locations
of specific dynamical orbital resonances in spiral galaxies has seen
much progress
in recent years. Research has focussed on two classes of objects: grand
design
spirals by the Elmegreens, and ringed galaxies by myself. The paper by
Elmegreen and Elmegreen
29
summarizes how to recognize the primary orbit resonances in
a relatively typical grand design spiral, NGC 1566. For this purpose,
purely
morphological methods guided by expectations from spiral structure
theory are
used. The features considered are spiral arm kinks, gaps, spurs,
bifurcations,
endpoints to star formation ridges, dust-lane crossover points, interarm
star
formation, and the ends of a weak bar. If consistency can be found
between the
positions of these features and those inferred for specific resonances from
a rotation curve, then the pattern speed of the wave can be derived with
reasonable
confidence. However, even in an extreme grand design case like NGC 1566,
the resonance features are very weak. It takes a great deal of tenacity,
for example,
for the reader to study and identify clearly all of the features summarized
in Table 1 of Elmegreen and Elmegreen
29.

Ringed galaxies refer to normal galaxies
classified in the de Vaucouleurs revised Hubble system with the symbols
(R)SB(r),
(R')SB(rs), (R')SAB(s), (R)SAB(r,nr) etc., that is, objects which have
inner,
outer, or nuclear rings or pseudorings. These rings are believed to
define the
locations of specific orbital resonances with a bar or oval, and if correct
they are much more obvious optical features with a direct link to
resonances
than some of the features seen in the best grand design spirals. Thus, they
provide a promising way of indirectly estimating pattern speeds of bars and
ovals, of which very little is known. At the moment, there is a great
deal of
evidence that the outer rings and pseudorings of SB and SAB galaxies
trace the
location of the outer Lindblad resonance, or OLR. This follows from
statistics of their shapes and orientations with respect to bars
10,
from their relative sizes with respect to inner rings
2
and most of all from their morphology
15.
The Catalogue of Southern Ringed Galaxies12
is designed specifically to understand
the link between rings and resonances, and has been the basis for the
studies of Buta
10
and Buta and Crocker
15.

A number of interesting findings have been
made concerning cluster galaxies. A photometric study of brightest
cluster members,
or "BCM's", including gE, D, and cD types (Schombert
51,
52,
53)
has led to a
refined and quantitative classification of these galaxies based on
luminosity profile shapes. Schombert has noted that the characteristic
extended envelopes
of cD galaxies are generally fainter than 10% of the night sky
brightness and are not readily seen on PSS prints. Thus, the rather
shallow luminosity profiles
of cD's is what led to their recognition, in addition to their central
location in clusters. It is the existence of a true extended envelope
that distinguishes the cD from the D class.

As emphasized by Sandage and Binggeli
47 (=SB),
the Virgo Cluster contains galaxies of virtually every known
morphological type.
Of particular interest has been the identification in Virgo of dwarf S0, or
dS0 galaxies, which morphologically are like S0's but which are of
considerably
lower luminosity and surface brightness than normal S0's (see also Binggeli
and Cameron
4).
Most of the galaxies in Virgo fainter than B 14 appear to
be dwarf E, or dE, systems. SB emphasize that the "great void" in
luminosity below Sa, Sb, and Sc types is real - there are no convincing
cases of dSa, for
example. This confirms that the Hubble sequence is largely defined by giant
galaxies. However, although no examples of dSa or dSb were found in
Virgo, a promising example was found by van den Bergh
63
in the compact, apparent elliptical
galaxy NGC 3928, a member of the Ursa Major Cloud of galaxies.

The luminosity
class system of van den Bergh has been extended to classes V-VI and VI
by Corwin
(see introduction to RC3) to allow for a greater range of apparent
surface brightnesses
seen among dwarf and late-type galaxies on the SRC-J sky survey. The RSA
luminosity
class system was also refined by SB to allow for a greater apparent
range of surface brightnesses seen among Im galaxies in the Virgo
Cluster. Among the
galaxies classified as Im V by SB are "huge" Im types having significant
diameters
(up to 10 kpc) and peak central surface brightnesses less than 10% of night
sky in blue light. These are accompanied by similar huge dE systems. The
data from a variety of sources of luminosity classes have been compared
and combined in RC3
24.

Van den Bergh, Pierce, and Tully
65
(=BPT) have discussed the classification
of 231 Virgo Cluster galaxies from CCD images. They propose a revision
to the classification system of van den
Bergh 62
to include Sd and Sm types, and demonstrate that the accuracy of luminosity
classification is improved on digital images
((MTB)
0.7 mag) compared to
classifications based on photographic plates published in the RSA
((MTB)
1 mag). Of particular interest in this work was the identification of a
possible
new class of galaxies, called "Virgo types." These galaxies have fuzzy
outer regions and active star formation in their bulges or inner disk
regions, and
constitute 43% of 88 Virgo cluster spirals. In contrast, BPT find that
the Ursa
Major cluster includes only 2 "Virgo types" out of 35 spirals,
suggesting real
differences. BPT suggest that the early "Virgo types" represent a mild form
of the Butcher-Oemler effect that persists at zero redshift.

In a study of the
HI and optical properties of cluster galaxies, Bothun, Schommer, and
Sullivan
6
identified a class of red, HI-rich, low surface brightness spirals. A
sample of these objects is compiled by Schommer and Bothun
54,
and two extreme examples
of the class, NGC 3883 and UGC 542, were studied by van der Hulst et al
66.
The types of these galaxies range from Sa to Sc in Schommer and Bothun
54,
and NGC 3883 is quite distinctive for its size and appearance in
Abell 1367. Van der Hulst et al
66
interpret these galaxies in terms of a threshold HI surface density
for star formation and possible interrupted star formation activity or
an altered IMF.

An important serendipitous finding from a study of a field of the Virgo
Cluster was an object dubbed "Malin 1"
7.
This galaxy appears small enough on
PSS prints that it did not make inclusion into the UGC
42.
However, on amplified
deep IIIa-J plates, Malin 1 shows an extended, low surface brightness
disk surrounding
a small bright core. The object is not a member of the Virgo Cluster (it is
20 times as distant)
and is now recognized as a new class of giant, HI
rich, low surface brightness disk galaxies the likes of which had not
been appreciated
before. The properties of Malin 1 are further summarized and described
by Impey and Bothun
36,
and a second example of the class was reported by Bothun et al
8.
These objects are now interpreted as disk galaxies whose HI surface density
is so low that they evolve only slowly.

The study of interacting galaxies has
led to the recognition of several new morphologies. Polar ring galaxies
69 are
believed to be cases where a small satellite has been disrupted into a
polar orbit around an S0. Hoag-type ringed galaxies
55
may be related cases where the
central object is an EO system. X-galaxies are edge-on S0's displaying a
distinct
X-shape across the center that may also be related to polar ring
galaxies
68.
"Ocular" galaxies are interacting galaxies displaying an "oval-apex"
structure resembling an eye
32.
The latter objects are particularly interesting, because
they represent a type of bar not distinguished within the Hubble system.
A key feature is a double arm on one side, as illustrated in Figure 1 of
Elmegreen et al
32.

Of particular interest to students of spiral structure is the discovery
of a leading spiral arm in the interacting galaxy NGC 462216.
The arm was first
noticed by Byrd on a well-known commercially available photograph published
in Shu
56.
The galaxy is of type SA(r)ab and shows two major outer arms that
wind clockwise, but inside the inner ring a single arm winding in the
opposite
sense is present. Since the "discovery" photograph was taken in blue light,
Buta, Crocker, and Byrd
15
(=BCB) re-observed the galaxy in the Cousins I-band
to test whether the arm is stellar or an artifact of dust. The leading
arm was
found to be a clear feature in the galaxy's old disk population. The
fact that
only a single leading arm is observed in this case, rather than two, is
strong evidence that the arm was generated by a tidal interaction, as
discussed in detail by BCB.

The widespread use of high quality CCD's, especially the large
format TEK CCD's at KPNO and CTIO, has greatly increased the number of
large-scale
images available for classifying galaxies. What is particularly
important is
that a typical modern CCD can provide in a short amount of time images that
are deeper in limiting surface brightness than the SRC-J sky survey, and
yet
still provide detailed information on the central regions of
galaxies. Thus,
they bypass the main problems of direct prime focus or Schmidt plates
and have
the potential of adding greatly to our knowledge of morphology. It is
also clear
that recent advances in infrared detectors make the development of a
classification
system in the 1-3 µ wavelength range a real possibility. The
advantages of using
near infrared images to type galaxies are their increased sensitivity to
the dominant old stellar populations, which tends to enhance the
visibility of features
such as bars and bulges. The young component of galaxies which dominates
blue
light images for many spirals as well as dust will be less prominent and
therefore
not important for typing purposes. The number of "cells" required to
classify
galaxies should therefore decrease somewhat. However, going to the infrared
will not change the pitch angle of spiral arms or the relative sense of the
Hubble sequence. What is clear is that the number of "non-barred"
galaxies will probably decrease, as can be gathered just the ESO-B and
ESO=R sky survey charts.

Finally, in the future some catalogues of galaxies will probably include
automatic classification
1,
58.
This is an approach still under development, owing to the
difficulty of defining some aspects of morphology, but once a
satisfactory methodology
is achieved, it has the potential of providing more consistent
classifications than might be achieved visually.